Sedimentary Characteristics and Oil Control of Chang 7 Member in Zhidan Area, Ordos Basin

Deep-water gravity flow deposits are widely developed in the Chang 7 member of the Triassic Yanchang Formation in the Ordos Basin.Due to its complex formation mechanism and many types, it is a great challenge to predict its development scale and distribution law.Taking the Chang 7 Member in Zhidan area as an example, on the basis of fine core description, through sedimentary structure, grain size analysis and logging curve, the types and characteristics of sedimentary facies, logging facies and distribution of Chang 7 Member were studied.The results show that:(1)The Chang 7 member in Zhidan area develops delta front subfacies and semi-deep lake-deep lake subfacies, including underwater distributary channel, interdistributary bay, sliding-slump rock, sandy debris flow deposition, turbidite and semi-deep lake-deep lake mud.(2)The grain size probability accumulation curve of Chang 7 member develops one-stage, two-stage and three-stage. The one-stage mainly indicates turbidity current deposition, and the two-stage and three-stage mainly indicate underwater distributary channel deposition. (3)There are many types of sedimentary structures in the Chang 7 member. There are a large number of slump structures or heavy load molds in the sliding-slump rocks. The sandy debris flow deposits mainly develop massive bedding and mud-coated gravel structure. Turbidite common sand-mud frequent rhythmic interbedding, Bouma sequence and flame-like structure.(4)The delta front subfacies are distributed in the northeast, and the underwater distributary channel microfacies are developed. From Chang 73 to Chang 71, its distribution range gradually increases. The semi-deep lake-deep lake subfacies are distributed in the southwest, and the sandy debris flow is generally developed. From Chang 73 to Chang 71, its distribution range gradually decreases.This study can provide geological basis for the selection of sweet spot areas for shale oil exploration in the Chang 7 member of Zhidan area.